This invention relates to the field of measuring overall image resolution capability in optical and retina systems such as a camera and its film and processing.
The precise definition and measurement of image resolving capability is of material importance in the highly demanding optical arts as aerial and space photography, microphotography, and night vision image enhancement. In such arts the ability to satisfactorily reproduce an image is often limited by both the ultimate capabilities of the optical components such as lenses and prisms and by the retina media involved. Military reconnaissance and mapping operations in particular usually require the best resolved imaging obtainable in the optical art in order that small objects in a large area photograph appear well defined even during highly magnified examination.
In view of the constant evolution and improvement occurring in the optical art, it is also desirable for optical system characterization purposes to have accurate and as much as possible, objective arrangements for measuring the resolution ability of a particular optical system--both the optical components of a system and the combination of particular optical components with prospective retina or, for example, photographic films. As suggested by author G.C. Brock in the technical article incorporated by reference below, the ability to relate a particular optical system to a given set of parameters is useful in both the technical characterization of, for example, new optical systems and in the identifying of desired performance levels in a vendor-purchaser setting.
Several arrangements for evaluating resolution performance of an optical system are known in the optics art. One of these arrangements, the combination of the objectively determined modulation transfer function (MTF) and the aerial image modulation (AIM) curves is particularly attractive for optical system resolution evaluation because of the variety of arrangements by which the MTF can be evaluated, and because of the mathematical utility of evaluating synthesized optical systems from the cascaded MTFs of the component parts--even including expected optical disturbance factors.
The mathematic basis for the MTF is known in the art and is, for example, discussed in the textbook Image Science authored by J. C. Dainty and R. Shaw and published in 1974 by Academic Press Incorporated of New York, NY; the disclosure of the Dainty and Shaw text is hereby incorporated by reference into the present specification. The MTF for a particular optical component can, in fact, be estimated by several different arrangements, including its Heaviside edge function response, with sinusoidal pattern based evaluation arrangements and by the arrangements disclosed in prior patents identified below herein. A Heaviside MTF estimation is generally based on the spatial frequency content of an optical edge image and is mathematically describable by the quotient of sine wave output modulation divided by sine wave input modulation.
The MTF, once determined, is actually an operator function which can be applied to spatial frequency spectra to predict an overall optical system's response. The MTF is, of course, most useful where information of known and limited spectra is being processed and therefore requires that most real-life scenes be simplified or approximated as to spectral content for its theoretically proper use.
A threshold function such as the Aerial Image Modulation (AIM) curve, otherwise known as a demand curve, or modulation detectability curve is, on the other hand, a measure of human ability to resolve pattern information. Generally, a determination of the AIM curve involves optical image disturbances occurring at low image contrast and varying image pattern frequency. The AIM curve is generally considered to be the threshold of resolving power measured at the 50% resolution level for the combination of an optical system, film, and film processing. Laboratory procedures for generating AIM curve threshold data are described, for example, in the publication "Photographic Emulsion Threshold Functions Study" (AFAL-TR-76-19), drafted by C. Rodriguez-Torres and R. B. Summers in March 1976 and based on work performed at the Air Force Avionics Laboratory, Wright-Patterson AFB, Ohio. The Rodriguez-Torres and Summers publication is hereby incorporated by reference into the present specification.
The combination of MTF and AIM curves has been shown to be a reliable and accurate method for predicting the resolution of a photo-optical system and predictions based on the combination of these curves have been shown to closely correlate with estimates of the previously used concept of resolving power. Such results are reported in the article "Determining Resolution of Photo-Optical Systems", written by M. A. Berkovitz, and appearing in the February/March 1969 issue of the publication Image Technology. The disclosure of the Berkovitz Image Technology article is also hereby incorporated by reference into the present specification.
The use of a combination of MTF and AIM characterizations in determining the limiting resolving power for a military reconnaissance system is described in the two publications "The Physical Aspects of Aerial Photography", written by G. C. Brock and published by Dover Publications, Inc. of New York, NY in 1967, and also in the technical article "Reflections on 30 Years of Image Evaluation", written by G. C. Brock in 1967 and published in Vol. 2, No. 5 of Photographic Science and Engineering. Both of these 1967 Brock publications are also hereby incorporated by reference into the present specification. In these publications the working convenience and also the theoretical weakness attending the common practice of converting MTF information into resolving power figures is discussed. The undesirable tendency of most commonly used test patterns to weight their characterization heavily in a limited spectral portion of the MTF is also discussed in these publications. Generally speaking therefore, MTF is in itself a more theoretically desirable measure of optical component performance than is resolving power, even though it is more cumbersome to use, especially in performance evaluation work. and is actually most applicable to sinusoidal input images. This desirability of sinusoidal images in determining the MTF curve of an optical system in combination with the common practice of employing square wave or high contrast bar patterns in determining the AIM curve is a point of doubtful theoretical substantiation in optical practice and is addressed by the capabilities of the present invention.
With respect to the determination of MTF through the approach of sinusoidal intensity images, it should be recognized that use of a Heaviside function image, an image comprised of a light to dark edge transition tends to be a determination of MTF by sinusoidal intensity modulated images. The Fourier series components used to describe a Heaviside edge function mathematically and into which an optical lens tends to decompose the Heaviside edge function are, of course, sinusoidal in nature.
Procedures for achieving MTF/AIM image analysis are also described in the publications "Modulation Transfer Analysis of Aerial Imagery" by E. L. Gliatti, published in Photogrammetria, Vol. 33, 1977, and in an article appearing in the Proceedings of the Society of Photo-optical Instrumentation Engineers, Vol. 137, and originating in the 28-29 March 1978 Washington, D.C. conference Airborne Reconnaissance, III", also authored by E. L. Gliatti. Such procedures are also discussed in the Berkovitz article referred to previously. The disclosure of the two Gliatti articles is also hereby incorporated by reference into the present specification.
Another optical analysis procedure is documented in the technical report AFAL-TR-74-218, titled "Photographic Systems Performance Analysis using Double-Annulus Targets" authored by R. Burke, W. Marshall and J. Kean and published by the Air Force Avionics Laboratory, Wright-Patterson AFB, Ohio in September 1974. The contents of the TR-74-218 report are also hereby incorporated by reference into the present specification.
A discussion of image quality estimators and also their capability for describing certain types of degradation often found in aerial photographs, e.g., relative motion of the photosensitive media and the image being photographed, is contained in a technical report titled "Investigation of Photographic Image Quality Estimators", authored by the inventor hereof, and dated April 1980. This report was prepared for the Air Force Aerospace Medical Research Laboratory, Aerospace Medical Division, Air Force Systems Command, Wright-Patterson AFB, Ohio, 45433, and is available from the National Technical Information Service at 5285 Port Royal Road, Springfield, VA 22161, under the designation of AFAMRL-TR-80-27. The image quality estimators report includes several pages of discussion and description concerning subjective image quality estimators and objective image quality estimators. The disclosure of the AFAMRL-TR-80-27 report is hereby also incorporated by reference into the present specification.
It is known in the art, therefore, to estimate the limiting resolution of military camera systems at the intersection of the modulation transfer function (MTF) curve, and the aerial image modulation (AIM) or modulation detectability curve. According to this practice the AIM curve may be subjectively derived from threshold data of photographically recorded three-bar groupings having decreasing bar width and made in accordance with military standard Mil. Std. 150A which is also hereby incorporated herein by reference. In this arrangement the three-bar groups resemble square waveforms, that is, waveforms having the appearance of a square wave, and the MTF curve is based on response to sine-wave patterns. It is, of course, also known that such square waveforms may be resolved into Fourier series components of a sinusoidal character. The use of square wave based waveforms for deriving the AIM curve has, however, been largely based on convenience and the absence of satisfactory equipment for generating accurate and controllabe sine wave patterns rather than on the theoretically desirable nature of such procedures.
The patent art discloses several inventions concerned with the measurement of optical resolution, and obtaining a modulation transfer function characterization of optical apparatus. Included in this art is the patent of H. T. Buschmann, U.S. Pat. No. 3,977,795, which concerns a method for determining the modulation transfer function. In the Buschmann patent a coherent, opticaly produced scatter of known noise distribution or known spatial frequency spectrum, is used in determining a modulation transfer function. The Buschmann invention determines only the real part of the modulation transfer function, the part used in assessing the quality of photographic films and papers, and teaches the use of optical energy of differing spectral content, i.e, different color, when required by the materials being assessed. The Buschmann invention employs an optical noise spectrum of wide but known frequency content for exciting the optical device under investigation and converts the optical noise signal output from the device into electrical signals which are subsequently examined for frequency and amplitude content; the results of the electrical signal examination are converted to MTF values through the use of two equations presented. The Buschmann apparatus is also compared with the use of sinusoidal test screens or sinusoidal gratings and found to provide increased speed and ease of determining the MTF. The disclosure of the Buschmann patent is also hereby incorporated by reference into the present specification.
The patent art additionally includes the invention of W. E. Flynt, in U.S. No. 4,060,328, which also concerns a system for measuring the modulation transfer function of an optical device. The Flynt apparatus employs a wide band excitation and converts the portion of this excitation transmitted by the unit under test to an electrical signal, a signal subsequently examined in a plurality of frequency segregated bands to determine the modulation transfer function. The Flynt apparatus is especially concerned with establishing an absolute 100% MTF reference level from which normalized MTF values can be based. In the Flynt invention, the low frequency component of the electrically converted optical signal is used as this 100% reference and the higher frequency components are determined as a percentage of this low frequency component signal. The low frequency component signal is based on a low optical spatial frequency, such as 0.75 line pairs per millimeter, and is presumed to be transmitted without attenuation by an image intensifier or other optical device under test and to be further processed without attenuation by a photomultiplier tube and electronic amplifier circuitry. The optical signal received at the photomultiplier tube of the Flynt invention is modulated by a device such as a motor-driven pattern wheel to provide at least one low frequency image component and one higher frequency image component for the photomultiplier. The disclosure of the Flynt patent is hereby also incorporated by reference into the present specification.
The patent art also includes U.S. Pat. No. 4,274,737, in the name of Bradford Howland, concerning test patterns for the evaluation of lenses. In the Howland patent, a lens test chart is fabricated from a plurality of different test patterns, including patterns having intensity variations that are periodic, sinusoidal, and of increasing spatial frequency. The Howland patent teaches against use of the modulation transfer function (except for noting that a lens response to a disclosed pattern approaches the MTF response) and does not consider the aerial image modulation or AIM curve. The Howland patent also teaches use of a three-dimensional chart having both linear and sinusoidal reflectance variations for use in the direct determination of modulation transfer function for a lens or optical component. The disclosure of the Howland patent is also hereby incorporated by reference into the present specification.
The patent art also includes U.S. Pat. No. 4,241,996 issued to one Sidney Weiser concerning an apparatus for measuring the quality of optical equipment through determining the modulation transfer function of the equipment. The Weiser apparatus is especially concerned with the measurement of modulation transfer function at low light levels where signal-to-noise ratio problems are expected and precise scanning is required. The Weiser invention teaches use of an optical pattern having bar elements of various spacings and each accompanied by a clock signal track. Ideally, the optical and electrical signals generated by the optical bar pattern are square wave in nature in the Weiser invention, however, under expected conditions of signal degradation by the device under test, rounded corner or sinusoidal appearing signals occur. The disclosure of the Weiser patent is also hereby incorporated by reference into the present specification.